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I took some water in a copper vessel and gave it a vigorous shake for approximately 10 mins when I realised that the temperature of the vessel is being raised a bit. But how is this possible, I mean if I shake it, I'm not providing any heat, although the water( in bulk) has some velocity( imparted by me ) but in a microscopic level each molecule will have the same initial K.E. ( I guess so ! ) so why will the temperature increase?If the K.E. increases then can I conclude that when we push a block( on a smooth floor) with some force through some distance it's temperature increases just because it's K.E. increases?

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  • $\begingroup$ Yes, kinetic energy will raise the temperature (very) slightly. More importantly however, your hands would warm up the vessel as you're holding it $\endgroup$
    – hdhondt
    Jan 3 '17 at 2:28
  • $\begingroup$ Yeah, but if I shake the vessel by insulating it from the outside ( thermally) by some means then change in temperature will be very slight,ryt? $\endgroup$ Jan 3 '17 at 2:31
  • $\begingroup$ Yes, if you insulate it, the rise in temp will be minimal - unless you shake it extremely vigorously. $\endgroup$
    – hdhondt
    Jan 3 '17 at 2:43
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FYI:

a standard high school experiment involves rotating a one metre long cardboard tube containing lead shot. After 100 completely rotations, the rise in temperature in the shot from falling 200 m can be readily measured, and used to find the specific heat of lead...

Here's simple on-line simulation http://www.gigaphysics.com/mech_equiv_lab.html

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It is said that Joule during his honeymoon measured the temperature of the water at the top and bottom of a waterfall and found that the temperature at the bottom was slightly more than the temperature at the top.

In falling the water lost gravitational potential energy and gained kinetic energy.
On hitting rocks at the bottom of the waterfall the kinetic energy due to the falling water became the kinetic energy of water molecules moving in random directions.
So the water molecules at the bottom of the waterfall had a greater random motion kinetic energy than those at the top, i.e. the water at the bottom of the waterfall was greater than at the top.

If all the molecules had the same increase in velocity there would be no increase in temperature.
It is the randomisation of the molecular motion which means that the temperature has risen.

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How about thinking the whole system as the copper vessel only? Water inside gives the pressure on the vessel, but no change in the volume. So, no work is done by water. However, friction loss in kinetic energy can be transported as heat. That is, work done by shaking is transported as heat to the vessel.

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When you shake vessel it receives kinetic energy, right. But it does not accumulate it - it moves with the same speed after 10 minutes. Where does this energy goes? It is dissipated by friction between vessel and water(maybe friction between vessel and your hand as well if your grip is slightly loose). Friction turn movement to heat - see how fire is started by rubbing two pieces of wood.

If you applied about 10-20 Watt to shaking this vessel(this is equivalent to raising 1-2 kg to height of one meter per second) with 100 g of water and it didn't lose any heat, then water would heat up 15-30 degrees(10 Watt * 600 seconds = 6 kJ = 0.1 kg of water * 4 kJ per degree per kg of water * 15 degrees).

Actually it would not heat up so much because heat goes to the air and your hand. That is why you feel vessel heating up.

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  • $\begingroup$ The fact that vessel is made of copper does not matter much. You would get basically same result with glass bottle. Maybe you would not feel heat if you used wood vessel - it does not transfer heat well, so heat would be mostly accumulated in water - it would heat up much more than water in copper vessel. $\endgroup$
    – Vashu
    Jan 3 '17 at 2:46
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It is important to realize that adding heat to a system and doing mechanical work on a system are indistinguishable processes from one another at the microscopic level. Essentially when you do work on the system you will raise its internal energy and hence the average kinetic energy of its constituency (you raise its temperature).

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